Field of the Invention
The invention concerns a method for controlling a magnetic resonance apparatus for magnetic resonance imaging, a magnetic resonance apparatus controlled by such a method, and a non-transitory, computer-readable data storage medium encoded with programming instructions that cause a control computer of a magnetic resonance apparatus to execute such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also referred to as a magnetic resonance tomography system, the body of a person to be examined, such as is planned in a scanner wherein the body of a patient, is exposed by the operation of a basic field magnet to a strong basic magnetic field, for example of 1.5 or 3 or 7 Tesla. In addition gradient fields are applied by a gradient coil arrangement. Radio-frequency radio-frequency pulses, namely excitation pulses, are radiated by a suitable antenna arrangement, which cause nuclear spins of specific atoms that are resonantly excited by these radio-frequency pulses to be flipped by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. During the relaxation of the nuclear spin, radio-frequency signals, so-called magnetic resonance signals, are emitted, which are received by suitable reception antennas, and are then further processed. The desired image data can be reconstructed from the raw data thus acquired.
For a specific magnetic resonance measurement, a specific magnetic resonance sequence, also known as a pulse sequence, is to be radiated, which includes a sequence of radio-frequency (RF) pulses, for example excitation pulses and refocusing pulses, as well as gradient pulses coordinated to match the RF pulses, on different gradient axes in different spatial directions. Readout windows matching these pulses in time are set, which predetermine the periods of time in which the induced magnetic resonance signals are detected.
During the acquisition of such raw data, for the homogeneity of the basic magnetic field in an examination volume of the scanner is of great significance. Even small deviations in the homogeneity can result in large deviations in a frequency distribution of the nuclear spin, so that the acquired magnetic resonance data are of lower quality.
Adjustments, which are specific for the examination object from which the magnetic resonance signals are detected, are typically carried out before a magnetic resonance measurement. A typical adjustment is a frequency adjustment in which a resonant frequency is established in an examination volume of the magnetic resonance measurement. Usually in such cases the resonant frequency is established from protons bound in water. The frequency adjustment is of advantage because the examination object, which is positioned in the patient receiving area of the scanner, can disturb the basic magnetic field in the scanner, and thus can influence the resonant frequency globally and/or locally. By the frequency adjustment, a system frequency of the scanner can be tuned to the examination object. The frequency adjustment in such cases can be an adjustment of a frequency for a radiation of the radio-frequency pulses and/or the read-out of the magnetic resonance signals. In this way the frequency adjustment can insure that the magnetic resonance data acquisition takes place at the respective Larmor precession frequency of the protons.